refactor oauth2 plugin

[yu199195]

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #4624 (1676a11) into master (13cf214) will increase coverage by 0.16%.
The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master    #4624      +/-   ##
============================================
+ Coverage     67.51%   67.67%   +0.16%     
- Complexity     7959     7961       +2     
============================================
  Files          1096     1094       -2     
  Lines         31311    31207     -104     
  Branches       2822     2812      -10     
============================================
- Hits          21140    21120      -20     
+ Misses         8517     8430      -87     
- Partials       1654     1657       +3     

Impacted Files	Coverage Δ
.../org/apache/shenyu/plugin/oauth2/OAuth2Plugin.java	75.00% <75.00%> (-19.45%)	⬇️
...arter/plugin/oauth2/OAuth2PluginConfiguration.java	100.00% <100.00%> (+35.71%)	⬆️

... and 36 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more